Valves are used in numerous applications across a number of industries, primarily for flow/pressure control purposes. For example, in the oil and gas industry valves are used extensively in both upstream and downstream applications. Valves may be employed in a downhole environment, such as in sub-surface safety valves (SSSV), downhole chokes, drill stem test valves and the like. Also, valves are typically used in topside applications, such as in wellhead equipment, for example in production trees, blow out preventers (BOPs) and the like.
When performing certain procedures on oil and gas wells, such as during workover or intervention operations, running completions, clean-up, abandonment and the like, it is necessary to utilise valves which are capable of isolating the formation from surface. Such valves may need to provide the capability to both contain fluids under pressure and also cut obstructions, such as wireline, coiled tubing, tool strings, or the like which extend through the valves. A variety of different valves are used for this so called shear and seal purpose, with the particular type selected dependent on variables such as the wellhead infrastructure and the nature of the wellbore operation.
In some instances where a marine riser is utilised to facilitate wellbore operations such as deploying completions or performing wellbore interventions, a so called landing string assembly is typically used, which extends inside the riser from surface to the wellhead, normally landed-out in a wellhead tubing hanger. This landing string may be used as a contained passage to permit fluids and/or equipment to be deployed from surface, and/or may be used to deploy wellbore equipment, such as completion strings, into the associated wellbore.
The landing string typically includes an upper section composed primarily of tubing, and a lower section which includes various valves for providing well control. For example, landing strings typically include a valve assembly called a subsea test tree (SSTT), which must provide a shear and seal functionality.
In many instances landing strings need to be sized and arranged not only to be deployed through a marine riser, but also to be accommodated within wellhead equipment, such as within BOP stacks. For example, the SSTT is typically located within the confines of the BOP, such that the outer dimensions of the SSTT are limited. Also, the axial extent of the SSTT needs to be such that, normally, it must be positioned between individual BOP rams, thus placing axial length size restrictions.
Further, the industry is increasing the requirements for such valves. Notably, emerging specifications such as ISO 13628-7 and API 17G are demanding that the structural integrity of such valves be improved to provide increased fatigue performance. The typical arrangement of current valves utilise much more of the available space to provide the existing functionality. For in-riser applications, there can be very little room to provide the additional functionality demanded by the industry codes.
Numerous valve designs exist, such as ball valves, flapper valves, ram valves, and the like. Each valve design has associated advantages and disadvantages, and often the particular design selected is very much dependent on the required application.
Ram valves, such as might be used in BOPs, have good cutting and post cut sealing capabilities, but typically require large projecting actuators which restricts their application, for example precluding the possibility of through riser deployment.
Ball valves can be diametrically compact, and thus permit use in through riser deployment applications. However, where ball valves also must have a cutting capability, post-cut sealing is often not guaranteed, for example as the cutting edge of the ball valve may be deformed, such that during the process of closing/opening the deformed cutting edge damages, for example by scoring, a cooperating sealing surface of a valve seat. Also, ball valves, particularly those used in SSTTs normally have associated internal linear actuators, which requires increased axial length, which can limit their ability to be installed in certain BOP stacks. Also, such internal actuators typically utilise elastomer type seals, which can suffer in the high pressures and temperatures normally associated with wellbores.
Flapper valves are generally accepted as being simple, reliable and trusted, and are well established in permanent barrier roles, such as in SSSV applications. However, a significant limitation of flapper valves is their ability to cut objects. Further, the nature of current flapper valves is such that their sealing surfaces are normally exposed to fluid flow when they are open, exposing them to damage with a possible compromise in sealing integrity when closed.